The viscosity of natural rubber increases (gelation occurs) during production, storage, and transport. The reason is thought to be that carbon-heteroatom bonds in the isoprene chain (aldehyde groups and the like) cross-link due to reacting with proteins, amino acids, and the like in the natural rubber, leading to gelation. This mechanism, however, has not been definitively explained.
This gelation is problematic, as it results in a degradation in the workability of the natural rubber as well as in physical properties, such as carbon black dispersibility and the like, upon combination with additives to yield a rubber composition. In order to resolve these problems, before the compounding step, a mastication step is performed to unravel molecular aggregates of rubber with a shear force and cut molecular chains, thereby peptizing the gel produced in the natural rubber. The mastication step is normally performed in one or more stages, using a different masticator from the kneader, or by removing the natural rubber after the mastication step and then reinserting the natural rubber into the kneader for the compounding step. In the mastication step, kneading is performed for longer than the preliminary compounding stage of the compounding step, the purpose of which is to increase the dispersibility of the additives. An alternative is to use stable viscosity natural rubber, yielded by adding a viscosity stabilizer to natural rubber at the time of production to suppress an increase in viscosity. For example, JP H6-256570 A (PTL 1) discloses the production of stable viscosity natural rubber yielded by adding a particular monohydrazide compound to natural rubber.